istry, despite the fact that it has applications across almost the whole breadth of
chemistry. The main advantage in conducting sonochemical experiments is that it
is cheap to get started in the field, for example, in Romania after Ceaucescu’s reign
scientists carried out organic reactions using cheap ultrasonic baths as their source
of radiation.
Sonochemistry is the research area in which molecules undergo chemical reaction due to the application of powerful ultrasound radiation (20 KHz–10 MHz) [4].
The physical phenomenon responsible for the sonochemical process is acoustic
cavitation. Let us first address the question of how 20 kHz radiation can rupture
chemical bonds (the question is also related to 1 MHz radiation), and try to explain
the role of a few parameters in determining the yield of a sonochemical reaction,
and then describe the unique products obtained when ultrasound radiation is used
in materials science.
A number of theories have been developed in order to explain how 20 kHz sonic
radiation can break chemical bonds. They all agree that the main event in sonochemistry is the creation, growth, and collapse of a bubble that is formed in the
liquid.
The first question is how such a bubble can be formed, considering the fact that
the forces required to separate water molecules to a distance of two van-der Waals
radii, would require a power of 10
5 W cm
À1 [4]. On the other hand, it is well
known that in a sonication bath, with a power of 0.3 W cm
À1 [4] water is already
converted into hydrogen peroxide. Different explanations have been offered; they
are all based on the existence of unseen particles, or gas bubbles, that decrease the
intermolecular forces, enabling the creation of the bubble. The experimental evidence for the importance of unseen particles in sonochemistry is that when the
solution undergoes ultrafiltration, before the application of the ultrasonic power,
there is no chemical reaction and chemical bonds are not ruptured.
The second stage is the growth of the bubble, which occurs through the diffusion of solute vapor into the volume of the bubble. The third stage is the collapse of
the bubble, which occurs when the bubble size reaches its maximum value.
From here on we will adopt the hot spot mechanism, one of the theories that
explain why, upon the collapse of a bubble, chemical bonds are broken. The theory
claims that very high temperatures (5,000–25,000 K) [5] are obtained upon the
collapse of the bubble. Since this collapse occurs in less than a nanosecond [5, 6],
very high cooling rates, in excess of 10
11 K s
À1 , are obtained. This high cooling rate
hinders the organization and crystallization of the products. For this reason, in all
cases dealing with volatile precursors, where gas phase reactions are predominant,
amorphous nanoparticles are obtained. While the explanation for the creation of
amorphous products is well understood, the reason for the nanostructured products is not clear. One explanation is that the fast kinetics do not permit the growth
of the nuclei. If, on the other hand, the precursor is a non-volatile compound, the
reaction occurs in a 200 nm ring surrounding the collapsing bubble [7]. In this
case, the sonochemical reaction occurs in the liquid phase. The products are
sometimes nanoamorphous particles and, in other cases, nanocrystalline. This depends on the temperature in the ring region where the reaction takes place. The
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
114
chemistry. The main advantage in conducting sonochemical experiments is that it
is cheap to get started in the field, for example, in Romania after Ceaucescu’s reign
scientists carried out organic reactions using cheap ultrasonic baths as their source
of radiation.
Sonochemistry is the research area in which molecules undergo chemical reaction due to the application of powerful ultrasound radiation (20 KHz–10 MHz) [4].
The physical phenomenon responsible for the sonochemical process is acoustic
cavitation. Let us first address the question of how 20 kHz radiation can rupture
chemical bonds (the question is also related to 1 MHz radiation), and try to explain
the role of a few parameters in determining the yield of a sonochemical reaction,
and then describe the unique products obtained when ultrasound radiation is used
in materials science.
A number of theories have been developed in order to explain how 20 kHz sonic
radiation can break chemical bonds. They all agree that the main event in sonochemistry is the creation, growth, and collapse of a bubble that is formed in the
liquid.
The first question is how such a bubble can be formed, considering the fact that
the forces required to separate water molecules to a distance of two van-der Waals
radii, would require a power of 10
5 W cm
À1 [4]. On the other hand, it is well
known that in a sonication bath, with a power of 0.3 W cm
À1 [4] water is already
converted into hydrogen peroxide. Different explanations have been offered; they
are all based on the existence of unseen particles, or gas bubbles, that decrease the
intermolecular forces, enabling the creation of the bubble. The experimental evidence for the importance of unseen particles in sonochemistry is that when the
solution undergoes ultrafiltration, before the application of the ultrasonic power,
there is no chemical reaction and chemical bonds are not ruptured.
The second stage is the growth of the bubble, which occurs through the diffusion of solute vapor into the volume of the bubble. The third stage is the collapse of
the bubble, which occurs when the bubble size reaches its maximum value.
From here on we will adopt the hot spot mechanism, one of the theories that
explain why, upon the collapse of a bubble, chemical bonds are broken. The theory
claims that very high temperatures (5,000–25,000 K) [5] are obtained upon the
collapse of the bubble. Since this collapse occurs in less than a nanosecond [5, 6],
very high cooling rates, in excess of 10
11 K s
À1 , are obtained. This high cooling rate
hinders the organization and crystallization of the products. For this reason, in all
cases dealing with volatile precursors, where gas phase reactions are predominant,
amorphous nanoparticles are obtained. While the explanation for the creation of
amorphous products is well understood, the reason for the nanostructured products is not clear. One explanation is that the fast kinetics do not permit the growth
of the nuclei. If, on the other hand, the precursor is a non-volatile compound, the
reaction occurs in a 200 nm ring surrounding the collapsing bubble [7]. In this
case, the sonochemical reaction occurs in the liquid phase. The products are
sometimes nanoamorphous particles and, in other cases, nanocrystalline. This depends on the temperature in the ring region where the reaction takes place. The
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
114